1. Field of the Invention
The present invention relates to a method for depositing a metal having a high corrosion resistance and a low contact resistance against carbon to a separator of a unit cell for forming a fuel cell.
2. Description of the Related Art
As a fuel cell, there are a solid polymer type fuel cell, a phosphoric acid type fuel cell, a molten carbonate type fuel cell, an alkali type fuel cell, etc. These fuel cells use different electrolytes, but acquire electromotive force by an electrochemical reaction of hydrogen and oxygen. A fuel cell is formed by stacked unit batteries (cells) and a separator for electrically connecting cells and for separating a reaction gas fed to the cells disposed between adjoining cells.
As this separator, a carbon material has been mainly used. With carbon, however, the required separator shape has been obtained by a shaving from a preform or pressurized powder shaping or the like. Accordingly, the productivity was low and the manufacturing cost high. Further, a separator made of titanium has been partially used as a separator made of metal, but titanium is an expensive material, so there is a limit in lowering of costs.
Therefore, a separator using as a preform a metal such as stainless steel or aluminum, which are materials having a high productivity and a low cost and which enable of reduction a weight by making the sheet thickness smaller has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 10-228914 etc. When using a metal as a preform, in order to lower the contact resistance against the carbon electrode or carbon paper forming a current collector and then impart corrosion resistance, it is necessary to plate it with a precious metal. This plating must be done immediately after removal of a passive film unlike with usual plating. Therefore, in a case of a wet type, there was a problem in that many steps such as degreasing, washing, surface activation, washing, plating, washing, and drying had to be passed. The hardware per se became large in size in physical vapor deposition (PVD) and chemical vapor deposition (CVD).
An object of the present invention is to provide a method for depositing a metal having a high corrosion resistance and a low contact resistance against carbon to a separator for a fuel cell enabling provision of an inexpensive separator for a fuel cell by depositing a metal having a high corrosion resistance and a low contact resistance against carbon to the surface of a metal conveniently by simple equipment while using as a preform a metal such as stainless steel or aluminum as a material having a high productivity and low price and in addition capable of reducing the weight by making the sheet thickness thin.
According to the present invention, there is provided a method of depositing a metal having a high corrosion resistance and a low contact resistance against carbon on a separator for a fuel cell, the method comprising the steps of projecting to a separator of a unit cell for forming the fuel cell a solid plating material comprised of core particles having a higher hardness than the separator and coated with a metal having a high corrosion resistance and a low contact resistance against carbon so as to compulsorily deposit the metal coated on this solid plating material to the separator.
Preferably, a projection velocity of the solid plating material to the separator is made 20 to 100 m/sec.
Preferably, the projection of the solid plating material to the separator is performed by a flow of dry air. Alternatively, the projection of the solid plating material is performed by a rotating impeller. Alternatively, the projection of the solid plating material is performed by a flow of water. Alternatively, the projection of the solid plating material is performed by a flow of inert gas.
More preferably, the core particles of the solid plating material have a particle size of 30 to 300 xcexcm, a true specific gravity of 2 to 15, and a hardness of 400/2000 Hv.
Still more preferably, the core particles of the solid plating material are made of hard metal.
Still more preferably, the metal having a high corrosion resistance and a low contact resistance against carbon to be coated on the core particles of the solid plating material is a single metal or an alloy.
Still more preferably, the metal having a high corrosion resistance and a low contact resistance against carbon to be coated on the core particles of the solid plating material is at least one of gold, silver, copper, and nickel.
Still more preferably, the carbon contact resistance value is not more than 20 mxcexa9xc2x7cm2 at a contact pressure of at least 1 kgxc2x7f/cm2.